(1) Field of the Invention
This invention relates to electric current regulating junctions that can serve as electrically actuated switches.
(2) Description of the Prior Art
New developments in the semiconductor memory industry include activatable contacts by means of which semiconductor devices arranged in a regular array can be serially connected with conductors according to a predetermined pattern so that some devices in the array are active and others are not. Such a system is called a read-only memory (ROM) system since it is useful for screening computer input or output. The process of selectively connecting only those semiconductor devices in the array that are intended to be active is referred to as writing into the read-only memory. Generally, there is prepared a complete regular array of semiconductor devices, even though some will not be used in establishing the desired pattern, with a junction between each semiconductor device and at least one of its connecting conductors. Certain junctions are activated to make semiconductor devices active in the desired pattern. As the connective link between the conductor and the semiconductor device, the junction must be highly conductive to avoid build-up of excessive heat which is produced by electrical power dissipation in the junction and which can limit the packaging densities of semiconductor devices per unit volume.
In summary of the above, elements for densely packaged read-only memory arrays must contain junctions that can display low electrical resistance, either when first formed in contact with a semiconductor device or when activated to make a semiconductor device controllably active. Junction compositions must be thermally stable at temperatures commonly used to form ohmic alloy contacts of metal conductors to semiconductor devices in the same and neighboring elements in a read-only memory array when the junction is formed before the contacts are made.
For alterable read-only memory (AROM) use, the junction must be able to function as a switch to make a semiconductor device controllably active or inactive. For example, it must be able to change from low resistance to high resistance to deactivate the semiconductor device in order to correct errors made in writing into the memory or to make substantial changes in the pattern of the memory.
Switchlike devices which are made with mixtures of electrically conducting and insulating materials of the art may not be suitable for use as junction compositions because they do not afford sufficient range of electric current regulation, they present too much electrical resistance, they tend to revert spontaneously at ordinary temperatures in memory use from a low resistance state to a high resistance state, or they are not thermally stable enough to withstand processing conditions commonly used in microcircuit and integrated circuit technology. For example, such devices may not be able to withstand heating to temperatures at which alloy bonding of contacting conductors to semiconductor devices commonly takes place or they may in their conductive states show excessively high resistances.
Junction compositions which are prepared by the vapor phase deposition of pure metal directly onto a semiconductor substrate, such as silicon, may not be sufficient to establish satisfactorily low contact resistance values because of the potential interference of just a few monolayers of silicon dioxide which may be present on portions of the semiconductor substrate in a memory array. The metals which generally are employed for vaporization are those that can be heated after vapor deposition to form a metal alloy with a silicon substrate at reasonable temperatures, such as 350.degree.-450.degree. C., to insure long-lived ohmic contacts of acceptably low conductivity. Aluminum is an example of such a vaporizable metal.
A need exists for compositions that do not require extensive processing in preparing electric current regulating junctions.